The present invention generally relates to anemometers and more particularly to a hot film anemometer.
The present invention is related to a hot wire anemometer which has previously been used to measure air flow, air turbulence and boundry layers. The principal behind a hot wire anemometer is that if an electrically heated fine wire is placed in a gas stream its resistance is altered by the air cooling of the wire the degree of wire cooling is proportional to fluid flow.
Anemometers of various types have been used to measure the mass of air flowing into an engine. FIG. 1 is illustrative of a prior art hot wire anemometer comprising a flow tube A and a resistance wire B positioned in a relatively rectangular orientation within the tube A. This approach has a number of deficiencies such as the resistance of the wire will vary as dust or other particulates in the inlet air stream become attached to the heated wires. These particulates may also form an insulative covering about the wire diminishing its sensitivity. As an example, the dust may contain silica which upon being heated by the resistance wire covers the wire with a glass-like coating. Other deficiencies of this type of anemometer are that its sensitivity to reverse air flow is very high. That is, this type of anemometer cannot distinguish between forward and reverse air flow. During certain instances a particular amount of air is caused to flow out of the engine, such as during a backfire, and again past the resistance wire. This type of anemometer would indicate that this reverse flow of air is, in fact, air flowing into the engine. This anemometer contains no mechanism for prohibiting the reverse airflow or for discriminating same. Also, if the wire spans a sufficiently large area it is not representative of the average flow through the tube A. FIG. 2 is illustrative of a different type of automotive hot wire anemometer. In order to increase its sensitivity (gain) to flow, the anemometer C contains a restriction or venturi D in the flow path. Situated about the venturi is an annulus E comprising a plurality of openings F which communicate with a measurement tube I into which are received two resistance elements H.sub.1 and H.sub.2. As the main flow of air moves though the inlet tube G a pressure differential is created which draws air through a measurement tub I. The inlet of the measurement tube can be oriented to receive ram air or otherwise. As previously mentioned, this type of anemometer exhibits a higher sensitivity to fluid flow and also a reduced sensitivity to reverse flow. A third type of anemometer, while not illustrated, includes a central tube into which is received flat plate oriented parallel to the flow of fluid. Situated on the surface of a large substrate are resistive thick film elements forming part of the anemometers' measurement circuit. One benefit of this anemometer is that it is less sensitive to particulates because of the flat parallel orientation. However, this type of device does not employ a venturi and as such its sensitivity (gain) is low and its response time is poor because of the relatively massive substrate. Further, this type of device cannot discriminate between forward and reverse air flow.
In general, conventional devices such as those described above use a relatively low velocity air stream exposed to a sense resistor. Therefor, a very low flow rates the flow becomes laminar upsetting the heat transfer from the heated resistor.
It is an object of the present invention to provide an anemometer which solves the deficiencies in the prior art. It is a further object of the present invention to provide an anemometer comprising a low resistive thermal mass capable of rapidly responding to flow variations. A further object of the invention is to provide an airflow sensor in which the air flow proximate a sensing element is turbulent.
Accordingly, the invention comprises: an anemometer comprising: an orifice plate adapted to be disposed within a flow stream, including an orifice; first means, spaced from and disposed downstream of the orifice plate for insulatingly suspending a plurality of resistive elements within a portion of the flow stream exiting the orifice; the plurality of resistive elements comprising a first (reference or cold) resistive element conformally shaped to the orifice, and a second resistive element substantially circumferentially disposed about the first resistive element. The second resistive element comprising a heated sense resistor having a heat transfer rate which changes with flow. The anemometer may further include second means, in circuit with the plurality of resistive elements, forming a sensing network therewith for generating a signal indicative of the flow stream.
Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.